Carbon supported Pt (Pt/C) with various average particle sizes ranging from sub 3 nm to 6.5 nm were in situ prepared and characterized at the cathode of proton exchange membrane fuel cells (PEMFCs). A clear Pt particle size effect on both the catalytic activity for oxygen reduction reaction (ORR) and the durability of the electrocatalyst was revealed. With the Pt particle size increase, both the surface specific activity and the electrochemical stability of Pt/C improved; however, the mass specific activity of Pt/C is balanced by the electrochemical surface area loss. The reduced occupation of corner and edge atoms on the Pt surface during the Pt particle size increase is believed to weaken the adsorption of the oxygenated species on Pt, and thereafter releases more available active sites for ORR and also renders the Pt surface a stronger resistance against potential cycling. It is therefore proposed that by designing the Pt microstructure with more face atoms on the surface, cathode electrocatalyst with both improved activity and enhanced durability would be developed for PEMFCs. (C) 2011 Elsevier B.V. All rights reserved.